1. Field of the Invention
The present invention relates to the field of electroless plating of a substrate, and more particularly to a method for treating a surface to facilitate electroless plating of the surface.
2. Description of the Prior Art
Electroless plating of various substrates have been well known in the art. The application of various types of electroless coatings, such as copper, cobalt, nickel, gold or palladium, is typically accomplished in a bath by chemical reduction of the metal, rather than by application of an electric current. The primary difficulty has been to prepare the non-conductive substrate to assure adequate adherence of the plating to the surface. It has also been a problem that certain plating techniques are not suitable for all types of substrates.
Electroless plating generally requires a so-called activation or catalyzation step during which a substrate surface, to be electrolessly plated, has placed thereon a material, usually a metal salt. This metal salt is capable of reducing the plated metal from an electroless bath without the use of an electrical current. Such a material serves as a reduction catalyst, and usually comprises the salts of the precious metals, namely palladium, platinum, gold, silver, iridium, osmium, ruthenium and rhodium.
Typically, the conventional processes include a first stage of etching or "microcrazing" the surface with a strong acid, such as a chromic acid. The etching solutions may also include sulphuric acid and sometimes phosphoric acid. The material to be plated is immersed in the acid bath to render the surface hydrophilic, and also to provide a keying effect to promote adhesion between the surface and subsequent coatings.
A problem with conventional electroless metal plating of hydrophobic surfaces, such as organic polymer surfaces, is that the etching often leads to an undesirable amount of surface roughness. Also, the need to handle and dispose of highly acidic and/or caustic materials presents difficulties.
Following the acid etching, the component is thoroughly rinsed to remove all traces of chromic acid. This rinsing is important as the presence of hexavalent chromium ions in subsequent solutions acts a poison in them. If such ions are retained by adsorption on the surface, they prevent it from receiving a uniform deposition of subsequent coatings. Therefore, multiple rinse tanks and spray rinses are usually employed, often with an intermediate immersion in an alkaline or acid solution.
The second stage of conventional processes is the activation or sensitization stage. The purpose is to provide active nuclei of the catalyst metal adsorbed onto the surface of the substrate. This stage is one approach may include immersion into a stannous chloride solution followed by rinsing and immersion into an activating bath containing in solution a catalytic precious metal, usually palladium. The activator, typically palladium chloride, reduces the Pd++ on the surface of the article to Pd.degree..
Alternatively, the material may be immersed in a solution containing both the tin salts and the precious metal, the "activator", followed by rinsing and immersing into a so-called "accelerator", usually a dilute acid solution, to modify the chemical nature of the tin component of the adsorbed activator. More particularly, there is employed a highly acidic solution of a noble metal colloid, typically a palladium colloid, maintained in suspension by a protective colloid, i.e. stannic acid colloids. Because the colloidal palladium as well as the protective colloid are co-adsorbed by the substrate, the article is immersed in an acidic or alkaline accelerator solution to remove the protective colloid and expose the adsorbed noble metal.
After water rinsing of the surface, the material is usually immersed into an electroless plating bath containing copper or nickel. The active precious metal nuclei act as a catalyst to produce a thin conductive metal coating on the surface.
Various modifications of this general procedure have been proposed by the prior art in an effort to improve the process and the resulting plating. In U.S. Pat. No. 3,983,267, issued to Norris on Sept. 28, 1976, there is described a process similar to that above described. However, the process is modified by including a step immediately following the acid etch in which the material is immersed in a reducing acid solution, preferably containing phosphorous, hypophosphorous or ortho-, hypo- or pyrophosphoric acids, and preferably including their alkali metal and/or ammonium salts as buffers. This process is said to make electroless plating available for such materials as polyphenylene oxides, ABS polymers and polyolefins.
The use of a linking agent immediately following the acid etching stage is disclosed in U.S. Pat. No. 3,993,848, issued to Feldstein on Nov. 23, 1976. The substrate surface is contacted with a linking agent for the stannous ion either prior to or concurrently with the priming step utilizing aqueous solution containing stannous and copper ions. The use of the linking agent is said to permit the use of lower amounts of stannous and copper ions while achieving improved plating and primer usefulness.
Particular sensitizing solutions have also been described in the prior art. In U.S. Pat. No. 3,960,573, issued to Zeblisky on June 1, 1976, there is discussed a sensitizing solution comprising a precious metal and a stoichiometric excess of a Group IV metal which is capable of two valence states. The solutions are stabilized against precious metal separation by adding a Lewis Base, e.g., hydroquinone or hydroxylamine. An alternative sensitizing solution including a complex of a precious metal salt with dimethyl sulfoxide, such as PdCl.sub.2.2(CH.sub.3).sub.2 SO, together with a Group IV metal salt such as stannous chloride is described in U.S. Pat. No. 3,963,841, issued to Anschel et al. on June 15, 1976.
In U.S. Pat. No. 3,958,048, issued to Donovan et al. on May 18, 1976, there is described a method for electroless plating in which copper or nickel salts are converted for direct absorption onto the substrate surface. The purpose of this technique is stated to be an avoidance of the need to utilize noble metals which are relatively expensive and have other potential difficulties in use. The Donovan procedure employs a water soluble reactant capable of forming a water insoluble, catalytically active reaction product which is absorbable in the suspended state, provided by a water soluble organic suspending agent.
An alternative to the etching technique prior to activation is to incorporate and disperse an activating metal species such as palladium metal into the polymer surface either directly or through the use of an ink or resinous covering coat or layer. It has been found however that this incorporation does not lead to a continuous electroless metallization but on the contrary leads to a metallization having voids. This may be due to the fact that in these prior art techniques the dispersed palladium species is encapsulated by the polymer and is thus rendered dormant and must be revitalized to its catalytic state by being exposed to the surface. Such revitalization has been attempted by abrading the polymer surface or by etching the surface with mineral acids such as sulfuric or nitric acid. However, the need to conduct such an etching type of step significantly negates the expected advantage of seeding the metal into the plastic, which is intended in part to avoid the need to etch.
A procedure of this latter type is described in U.S. Pat. No. 4,035,500, issued to Dafter on July 12, 1977. In the Dafter patent there is disclosed a method which includes forming a dielectric coating containing an activating metal species on the surface of the substrate, and then reviving the metal species with chromic acid or ceric ammonium nitrate.
In U.S. Pat. No. 4,244,789, issued to Coll-Polagos on Jan. 13, 1981, there is described a method for metallizing materials which includes coating the substrate material with a hydrophilic composite material. The method is indicated as useful for producing metallized foams, embossing plates for reproduction of grains and textures, and decorative coatings for substrate materials. The hydrophilic composite material system includes a solvent, a film forming component, such as a vinyl chloride-vinyl acetate blend, and a hydrophilic component, such as polyvinylpyrrolidone. After application of the coating, the surface is contacted with water or a water solution of salts to produce a microporous surface by dissolving the water soluble, hydrophilic component.
An example of an electroless metal plating solution is described in U.S. Pat. No. 3,959,531, issued to Schneble et al. on May 25, 1976. The Schneble et al. composition includes, in combination, as ion of a metal whose electroless metal deposition is desired, a complexing agent for the ion, a reducing agent for said ion, a pH regulator, and less than about 25 parts per million of metal ions which have an oxidation potential greater than the oxidation potential of the ion of metal to be deposited.
The need for electroless plating of various substrates, particularly plastics, is continually growing. One application for this methodology is in the field of electromagnetic/radio frequency interference (EMI/RFI) coatings. The Federal Communications Commission has regulations that limit EMI/RFI for all digital electronic products that generate or use frequencies between 10 kHz and 1000 kHz, and include commercial, business or industrial products such as computers, cash registers, electronic typewriters and home products such as personal computers, video equipment, electronic games and calculators. This requirement has resulted at least in part due to the growing EMI/RFI from certain plastic unshielded electronic products that interfere with computers, pacemakers, radio/television, aircraft navigation and test instruments. Miniaturization of electrical devices has also stepped up sensitivity to small, spurious signals. As a result there is a present and increasing need for electroless plating methods which are suitable to a wide variety of substrates, and which are simple and reliable in operation.